Threaded unions are used to provide fluid-tight joints in fluid conduits. Threaded unions are held together by a threaded nut that is tightened to a required torque using a hammer or a wrench. In the oil industry, threaded unions are generally constructed using “wing nuts” and are commonly called “hammer unions” or “hammer lug unions”. Hammer unions are designed and manufactured in accordance with the specifications stipulated by the American Petroleum Institute in API 6A entitled “Specification for Wellhead and Christmas Tree Equipment”. Hammer unions are usually available in a variety of sizes (1″ to 12″) and a variety of pressure ratings (1000 psi to over 20,000 psi).
One substantial disadvantage of most prior-art threaded unions is that they rely on elastomeric seals for achieving a fluid-tight joint. Elastomeric seals are vulnerable to the extreme temperatures generated by fire. In the event that a fire erupts around a high-pressure conduit, the elastomeric seal in the threaded union may leak or fail completely which may exacerbate the fire if the leak permits combustible fluids to escape to the atmosphere.
While flanged unions are commonly used in well trees, pipelines and other high-pressure applications where temperature tolerant seals are required, flanged unions are relatively expensive to construct and time-consuming to assemble in the field. Metal ring gaskets are known for flanged unions, such as the BX ring gasket manufactured in accordance with API 6A. In operation, however, these BX ring gaskets are deformed beyond their yield strength and must be discarded after a single load cycle.
It is well known in the art that there is increasing pressure on the oil industry to produce hydrocarbons at a lower cost. Consequently, an interest has developed in utilizing wellhead equipment that is less expensive to construct and is more quickly assembled than prior art flanged unions. Threaded unions provide a good alternative to flanged unions from a cost standpoint because they are faster to assemble and less expensive to construct. However, due to safety concerns related to the lack of a reliable high-pressure metal-to-metal seal, use of threaded unions for well tree components and other high-pressure temperature tolerant applications has not been endorsed.
Therefore, it is highly desirable to provide an improved threaded union having a high-pressure metal-to-metal seal.